Synthesis and reactivity of 7,9-diphenyl-nido-carbaundecaboranes: Rearrangement processes in carbaplatinaboranes revisited

Article abstract of DOI:10.1039/a607523a

A number of synthetic routes towards diphenyl nido-carbaborane [7,9-Ph₂-7,9-nido-C₂B₉H₁₀] ^- have been elucidated. Base-induced degradation of 1,7-Ph₂-1,7-closo-C₂B₁₀H₁₀ afforded two isolable products [NEt₃H]-[7,9-Ph₂-x-OEt-7,9-nido-C₂B ₉H₉] (x = 3 1a or 2 1b), both of which contain an ethoxide-substituted cage. The structural identity of 1a was established by a single-crystal X-ray diffraction study. Fluoride-induced degradation gave a product assigned as [NBu₄][7,9-Ph₂-10-OH-7,9-nido-C₂B ₉H₉] 2, also studied crystallographically. Heating a solid sample of Cs[7,8-Ph₂-7,8-nido-C₂B₉H₁₀] in a sealed tube under vacuum for 4 h at 300°C afforded the target species [NEt₃H][7,9-Ph₂-7,9-nido-C₂B₉H ₁₀] 3, after cation metathesis. Deprotonation of 3 and reaction with [PtCl₂(PMe₂Ph)₂] afforded the new compound 1,1-(PMe₂Ph)₂-2,4-Ph₂-1,2,4-closo-PtC ₂B₉H₉ 4. Compound 4 has been characterised by multinuclear (¹H, ¹¹B and ³¹P-{¹H}) NMR spectroscopy as well as a single-crystal X-ray diffraction study. The isolation of this stable (in refluxing toluene) compound having a 1,2,4-MC₂B₉ cage architecture indicates that a hextuple-concerted diamond-square-diamond mechanism cannot be operating in the spontaneous isomerisation of 1,2-Ph₂-3,3-(PMe₂Ph)₂-3,1,2-closo-PtC ₂B₉H₉ below ambient temperature to afford 1,1-(PMe₂Ph)₂-2,8-Ph₂-1,2,8-closo-PtC ₂B₉H₉. The new compounds 1,1-(PR₃)₂-2,4-Ph₂-7-OEt-closo-1,2,4-PtC ₂B₉H₈ (R₃ = Me₂Ph 5a or Et₅ 5b), both fully characterised by multinuclear NMR spectroscopy, have also been synthesized.

Full text of DOI:10.1039/a607523a

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Synthesis and reactivity of 7,9-diphenyl-nido-carbaundecaboranes:
rearrangement processes in carbaplatinaboranes revisited†
Alan J. Welch and Andrew S. Weller *
Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, UK
A number of synthetic routes towards diphenyl nido-carbaborane [7,9-Ph2-7,9-nido-C2B9H10]Ϫ have been
elucidated. Base-induced degradation of 1,7-Ph2-1,7-closo-C2B10H10 afforded two isolable products [NEt3H]-
[7,9-Ph2-x-OEt-7,9-nido-C2B9H9] (x = 3 1a or 2 1b), both of which contain an ethoxide-substituted cage. The
structural identity of 1a was established by a single-crystal X-ray diffraction study. Fluoride-induced degradation
gave a product assigned as [NBu4][7,9-Ph2-10-OH-7,9-nido-C2B9H9] 2, also studied crystallographically. Heating a
solid sample of Cs[7,8-Ph2-7,8-nido-C2B9H10] in a sealed tube under vacuum for 4 h at 300 ЊC afforded the target
species [NEt3H][7,9-Ph2-7,9-nido-C2B9H10] 3, after cation metathesis. Deprotonation of 3 and reaction with
[PtCl2(PMe2Ph)2] afforded the new compound 1,1-(PMe2Ph)2-2,4-Ph2-1,2,4-closo-PtC2B9H9 4. Compound 4 has
been characterised by multinuclear (1H, 11B and 31P-{1H}) NMR spectroscopy as well as a single-crystal X-ray
diffraction study. The isolation of this stable (in refluxing toluene) compound having a 1,2,4-MC2B9 cage
architecture indicates that a hextuple-concerted diamond–square–diamond mechanism cannot be operating in the
spontaneous isomerisation of 1,2-Ph2-3,3-(PMe2Ph)2-3,1,2-closo-PtC2B9H9 below ambient temperature to afford
1,1-(PMe2Ph)2-2,8-Ph2-1,2,8-closo-PtC2B9H9. The new compounds 1,1-(PR3)2-2,4-Ph2-7-OEt-closo-1,2,4-
PtC2B9H8 (R3 = Me2Ph 5a or Et3 5b), both fully characterised by multinuclear NMR spectroscopy, have also been
synthesized.
We are currently interested in crowded carbametallaboranes
of the type 1-Ph-2-R-3-(µ-L)-3,1,2-MC2B9H9 (R = H, Me or
Ph; L = C5H5, C6H6 for example).2 When R = Ph steric conges-
tion between the adjacent phenyl groups results either in poly-
hedral distortion, leading to pseudocloso structures (e.g. a) in
which the cage-carbon connectivity has been broken, or in cage-
carbon isomerisation. An unusually facile example of the latter
is the sterically induced low-temperature (<20 ЊC) polyhedral
rearrangement of 1,2-Ph2-3,3-(PMe2Ph)2-3,1,2-closo-PtC2-
B9H9 3 to afford 1,1-(PMe2Ph)2-2,8-Ph2-1,2,8-closo-PtC2B9H9 (b
and c respectively).
We noted 3 that generation of the 2,1,8-PtC2B9 product was
probably inconsistent with the elegant hextuple-concerted
diamond–square–diamond (d.s.d.) mechanism for rearrange-
ments of icosahedral heteroboranes. In such a mechanism, the
C(1)᎐C(2) connectivity would be broken in the initial transition
state and would result in a species with a 1,2,4-PtC2B9 cage
architecture (d). This compound would not be expected to
have the severe intramolecular crowding between adjacent
phenyl groups present in 3,1,2-closo-PtC2B9, and thus could be
stable to further isomerisation. Thus, the independent isolation
of this 1,2,4-PtC2B9 species would argue strongly against the
d.s.d. mechanism operating in the above case.
In order to confirm this hypothesis, the synthesis of the 1,2,4-
PtC2B9 isomer was attempted directly starting from [7,9-Ph2-
7,9-nido-C2B9H10]Ϫ (note the change in conventional numbering
between closo and nido species). This paper describes a number
of attempted synthetic routes towards [7,9-Ph2-7,9-nido-
C2B9H10]Ϫ and the reaction between this carbaborane and
[PtCl2(PMe2Ph)2]. Also reported are the products of reactions
between [PtCl2(PR3)2] (R3 = Et3 or Me2Ph) and [NEt3H][3-OEt-
7,9-Ph2-nido-7,9-C2B9H9].
were carried out under a dry, oxygen-free dinitrogen atmos-
phere, using Schlenk-line techniques, although all the new
compounds reported are stable to the ambient atmosphere both
as solids and as solutions. The fluoride decapitation reactions
were performed as described recently by Wade and co-workers.4
All solvents were dried over the appropriate drying agents
immediately prior to use [CH2Cl2, CaH2; tetrahydrofuran (thf)
and diethyl ether, sodium wire–benzophenone; toluene, light
petroleum (b.p. 40–60 and 60–80 ЊC), sodium wire]. Chrom-
atography columns (3 × 15 cm) were packed with silica
(Kieselgel 60, 200–400 mesh). The compounds [PtCl2-
Experimental
General
6
(PMe2Ph)2],5 1,7-Ph2-1,7-closo-C2B10H10 and Cs[7,8-Ph2-7,8-
All experiments, apart from fluoride decapitation reactions,
nido-C2B9H10]7 (similarly to Cs[7-Ph-7,8-C2B9H11]7) were
† Steric effects in heteroboranes. Part 17.1
prepared as described previously.
J. Chem. Soc., Dalton Trans., 1997, Pages 1205–1212
1205

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Spectroscopy
IR (KBr disc): 3443 (vbr) (OH), 2539 (br) cmϪ1 (BH). NMR
[(CD3)2CO]: H, δ 7.38 (m, 4 H, Ph), 6.95–6.70 (m, 6 H, Ph),
1
Proton NMR spectra were recorded on a Brüker AC 200 spec-
trometer and H, 11B and 31P spectra on a Brüker DPX 400
1
3.25 (m, 8 H, NCH2), 1.70 (m, 8 H, CH2), 1.31 (m, 8 H, CH2)
and 0.87 [t, 12 H, CH3, J(HH) 8]; 11B, δ 3.67 (s, 1 B), 1.36 [d, 1
B, J(HB) 128], 0.79 [d, 1 B, J(HB) ca. 120], Ϫ0.70 [d, 1 B, J(HB)
141], Ϫ15.19 [d, 1 B, J(HB) 143], Ϫ18.30 [d br, 1 B, J(HB) 156],
Ϫ22.30 [d, 1 B, J(HB) 165], Ϫ31.50 [d, 1 B, J(HB) 143] and
Ϫ33.65 [d, 1 B, J(HB) 133 Hz]; 11B-{1H}, δ 3.67 (1 B), 1.36
(1 B), 0.79 (1 B), Ϫ0.70 (1 B), Ϫ15.19 (1 B), Ϫ18.30 (1 B),
Ϫ22.30 (1 B), Ϫ31.5 (1 B) and Ϫ33.65 (1 B).
spectrometer at 297 K in CDCl3 solutions unless otherwise
stated. Proton chemical shifts are reported relative to residual
protio solvent in the sample, 11B relative to external BF3ؒOEt2 at
128.0 MHz and 31P relative to external H3PO4 at 162.0 MHz.
Infrared spectra were recorded from CH2Cl2 or thf solutions,
using 0.1 mm NaCl solution cells on a Nicolet Impact 400
FTIR spectrophotometer, unless otherwise stated.
[NEt3H][7,9-Ph2-7,9-nido-C2B9H10] 3. The compound Cs-
[7,8-Ph2-7,8-nido-C2B9H10] (0.300 g, 0.717 mmol) was heated in
a sealed tube for 4 h at 300 ЊC. The resulting pale yellow solid
was dissolved in CH2Cl2 (40 cm3) and [NEt3H]Cl (0.15 g, 1.1
mmol) added to afford a white precipitate, which was filtered
off to give [NEt3H][7,9-Ph2-7,9-nido-C2B9H10] 3. Mass 0.21 g
(75%) (Found: C, 61.7; H, 9.95; N 2.9. Calc. for C20H36B9N:
Syntheses
[NEt3H][7,9-Ph2-x-OEt-7,9-nido-C2B9H9] (x = 3 1a or 2 1b).
The compounds 1,7-Ph2-1,7-closo-C2B10H10 (0.78 g, 2.63 mmol)
and KOH (0.36 g, 6.42 mmol) were dissolved in degassed etha-
nol (50 cm3) in an autoclave (150 cm3) equipped with a magnetic
stirring bar and flushed with dinitrogen. The autoclave was
heated to 200 ЊC for 18 h under autogenous pressure [ca. 12 bar
(ca. 1.2 × 106 Pa)]. Once cooled, CO2 was bubbled through the
solution for 1 h. The white, sticky, precipitate of K2CO3 that
formed was filtered off and the resulting pale yellow solution
reduced in vacuo to leave an oily, pale yellow solid. This was
redissolved in water (50 cm3) and [NEt3H]Cl (0.38 g, 2.76
mmol) in water (5 cm3) was added to afford an off-white pre-
cipitate, which was filtered off and recrystallised from CH2Cl2–
light petroleum (b.p. 60–80 ЊC) at 3 ЊC to afford white, needle-
shaped crystals of compound 1a. A second recystallisation of
the supernatant afforded a lower yield of pure 1a. Total mass
0.61 g, 54%. A third recystallisation precipitated a small
amount of white solid that was a mixture of 1a (ca. 20%) and 1b
1
C, 61.9; H, 9.9; N, 3.6%). NMR [(CD3)2CO]: H, δ 7.45 (m, 4
H, Ph) and 7.05–6.82 (m, 6 H, Ph); 11B, δ 0.72 [d, 3 B one
coincident, J(HB) ca. 140], Ϫ15.25 [d, 2 B, J(HB) 139], Ϫ18.31
[d br, 2 B, J(HB) 97], Ϫ28.40 [d, 1 B, J(HB) 135] and Ϫ32.80 [d,
1 B, J(HB) 134 Hz]; 11B-{1H}, δ 0.72 (3 B, one coincident),
Ϫ15.25 (2 B), Ϫ18.31 (2 B), Ϫ28.40 (1 B) and Ϫ32.80 (1 B).
1,1-(PMe2Ph)2-2,4-Ph2-1,2,4-closo-PtC2B9H9 4. The com-
pound [NEt3H][7,9-Ph2-7,9-nido-C2B9H10] (0.100 g, 0.253
mmol) was deprotonated by reflux for 18 h with 2.5 equivalents
of washed NaH in thf. This solution was added to a frozen
(Ϫ196 ЊC) solution of [PtCl2(PMe2Ph)2] (0.135 g, 0.253 mmol)
in thf (10 cm3). Warming to room temperature and stirring for
4 h resulted in an orange-brown solution. The solvent was
removed in vacuo and the residue taken up in CH2Cl2. Filtration
through a Celite pad afforded an orange-brown solution, which
was reduced in volume in vacuo to ca. 2 cm3. This was applied to
the top of a chromatography column, and elution with CH2Cl2–
light petroleum (b.p. 40–60 ЊC) (1:1) afforded a bright yellow
band. Recrystallisation from CH2Cl2–light petroleum at 0 ЊC
afforded bright yellow crystals of 1,1-(PMe2Ph)2-2,4-Ph2-1,2,4-
closo-PtC2B9H9 4, mass 0.61 g (32%) (Found: C, 54.0; H, 5.15.
Calc. for C30H41B9PtP2: C, 54.7; H, 5.55%). NMR: 1H, δ 7.46–
6.83 (m, 18 H, Ph), 5.97 (m, 2 H, Ph), 1.41 [d, 6 H, PMe, J(PH)
10, J(PtH) 42] and 0.72 [d, 6 H, PMe, J(PH) 10, J(PtH) 38];
11B,§ δ Ϫ4.49 (br, 3 B, one coincident), Ϫ11.69 (br, 2 B), Ϫ15.84
(br, 2 B), Ϫ17.86 (br, 1 B) and Ϫ19.37 (br, 1 B); 11B-{1H}, δ
Ϫ4.49 (3 B), Ϫ11.69 (2 B), Ϫ15.84 (2 B), Ϫ17.86 (1 B) and
Ϫ19.37 (1 B); 31P-{1H}, δ Ϫ15.86 [d, J(PP) 14, J(PtP) 4758] and
Ϫ20.00 [d br, J(PtP) 3792 Hz].
1
(ca. 80%). A H NMR spectrum of the mixture before any
recrystallisation showed that the ratio of 1a to 1b was approxi-
mately 9:1.
Compound 1a (Found: C, 59.5; H, 8.7; N, 3.0. Calc. for
C22H40B9NOؒ0.25CH2Cl2: C, 58.9; H, 8.9, N, 3.0%). NMR: 1H,
δ 8.55 (s br, 1 H, HNEt3), 7.51 (m, 4 H, Ph), 7.15 (m, 6 H, Ph),
3.67 [apparent q, 2 H, OCH2, J(HH) 6], 2.95 [q, 6 H, NCH2,
J(HH) 7], 1.18 [t, 9 H, CH3, J(HH) 7] and 1.07 [t, 3 H, CH3,
J(HH) 6]; 11B, δ 2.00 (sharp, 1 B), 0.64 (br, 1 B), Ϫ1.75 (br, 1 B),
Ϫ2.10 (br, 1 B), Ϫ17.13 (br, 1 B), Ϫ18.2 (br, 1 B), Ϫ22.37 (br, 1
B), Ϫ29.35 [d, 1 B, J(HB) 121] and Ϫ35.79 [d, 1 B, J(HB) 130
Hz]; 11B-{1H}, δ 2.00 (1 B), 0.64 (1 B), Ϫ1.75 (1 B), Ϫ2.10 (1 B),
Ϫ17.13 (1 B), Ϫ18.2 (1 B), Ϫ22.37 (1 B), Ϫ29.35 (1 B) and
Ϫ35.79 (1 B).
Compound 1b (isolated as a 1:4 mixture of 1a and 1b): NMR
1H, δ 7.56 (m, 4 H, Ph), 7.25–6.96 (m, 6 H, Ph), 3.59 [q, 1 H,
OCH2, J(HH) 7], 3.42 [q, 1 H, OCH2, J(HH) 7], 2.97 [q, 6 H,
NCH2, J(HH) 7], 1.14 [t, 9 H, CH3, J(HH) 7] and 0.93 [t, 3 H,
CH3, J(HH) 7 Hz]; 11B-{1H},‡ δ 0.53 (br, 2 B), Ϫ2.84 (br, 3 B),
Ϫ22.07 (br, 2 B), Ϫ30.62 (br, 1 B) and Ϫ39.05 (br, 1 B).
1,1-(PMe2Ph)2-2,4-Ph2-7-OEt-1,2,4-closo-PtC2B9H8 5a. The
compound [NEt3H][7,9-Ph2-3-OEt-7,9-nido-C2B9H9] (0.100 g,
0.258 mmol) was deprotonated by reflux for 18 h with 2.5
equivalents of NaH in thf. This solution was added to a frozen
(Ϫ196 ЊC) solution of [PtCl2(PMe2Ph)2] (0.135 g, 0.248 mmol)
in thf (10 cm3). Warming to room temperature and stirring for
2 h resulted in an orange-brown solution. The solvent was
removed in vacuo and the residue taken up in CH2Cl2. Filtration
through a Celite pad afforded an orange-brown solution, which
was reduced in volume in vacuo to ca. 2 cm3. This was applied to
the top of a column, and elution with CH2Cl2 afforded a bright
yellow band. Recrystallisation from CH2Cl2–light petroleum
(b.p. 60–80 ЊC) afforded bright yellow microcrystals of 1,1-
(PMe2Ph)2-2,4-Ph2-7-OEt-1,2,4-closo-PtC2B9H8 5a, mass 0.060
g (29%) (Found: C, 46.9; H, 5.75. Calc. for C32H45B9OP2Pt: C,
48.0; H, 5.7%). NMR: 1H, δ 7.62–6.96 (m, 18 H, Ph), 6.09 (m, 2
H, Ph), 3.67 [d of q, 1 H, OCH2, J(HH) 10, 7], 3.25 [d of q, 1 H,
OCH2, J(HH) 10, 7], 1.46 [d, 3 H, PMe, J(PH) 11, J(PtH) 44],
1.44 [d, 3 H, PMe, J(PH) 11, J(PtH) 43], 1.10 [t, 3 H, CH3,
[NBu4][7,9-Ph2-10-OH-7,9-nido-C2B9H9] 2. The compound
1,7-Ph2-1,7-closo-C2B10H10 (0.300 g, 1.01 mmol) was dissolved
in thf (20 cm3) and [NBu4]F (5 cm3 of a 1.0 mol dmϪ3 solution
᎐
in thf ᎐ 5.0 mmol) was added. The solution was heated to reflux
᎐
overnight, after which the characteristic closo B᎐H stretch at
2600 cmϪ1 had been replaced by one at 2545 cmϪ1 characteristic
of a nido-C2B9 cage. Dichloromethane (100 cm3) was added, and
the resulting solution washed with water (3 × 50 cm3). The
organic layer was dried over MgSO4, following which removal
of the solvent afforded a white solid, which on recrystallisation
from hot ethanol gave white needles of [NBu4][7,9-Ph2-10-OH-
7,9-nido-C2B9H9] 2, mass 0.40 g (78%). Positive-ion FAB mass
spectrum: m/z 787 {[NBu4]2[C2B9H10OPh2]} (Found: C, 65.7; H,
10.4; N, 2.5. Calc. for C30H56B9NO: C, 66.3; H, 10.3; N, 2.6%.
‡ The resonances at δ Ϫ30.62 and Ϫ39.05 were observed as doublets in
the 11B NMR spectrum, J(HB) 116 and 135 Hz respectively; three over-
lapping resonances can be inferred from the asymmetry of the peak at
δ Ϫ2.84.
§ Broadness of peaks means that J(BH) was not resolved.
1206
J. Chem. Soc., Dalton Trans., 1997, Pages 1205–1212

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J(HH) 7], 0.90 [d, 3 H, PMe, J(PH) 10, J(PtH) 28] and 0.77 [d, 3
H, PMe, J(PH) 10, J(PtH) 28]; 11B-{1H},§ δ 0.47 (1 B), Ϫ4.61
(2 B), Ϫ6.50 (1 B), Ϫ15.67 (3 B) and Ϫ19.49 (2 B); 31P-{1H}, δ
Ϫ16.39 [d, J(PP) 14, J(PtP) 4744] and Ϫ20.05 [d br, J(PP) 14,
J(PtP) 3805 Hz.]
degradation (KOH–EtOH) of 1,7-Ph2-1,7-closo-C2B10H10.
Reactions in an autoclave between 160 and 200 ЊC afforded
only two isolable products, neither of which was the anticipated
[7,9-Ph2-7,9-nido-C2B9H10]Ϫ anion. Separation of these two
products by fractional crystallisation afforded a major (1a) (ca.
1
54% yield) and a minor (1b) (ca. 5% yield) product. The H
1,1-(PEt3)2-2,4-Ph2-7-OEt-1,2,4-closo-PtC2B9H8 5b. In an
identical manner to that described for compound 5a;
[NEt3H][7,9-Ph2-3-OEt-7,9-nido-C2B9H9] (0.070 g, 0.162 mmol)
was deprotonated and treated with [PtCl2(PEt3)2] (0.082 g,
0.163 mmol). Chromatography afforded a fast moving minor
yellow band which contained no B᎐H (by IR) and a subsequent
major yellow band. Recrystallisation from CH2Cl2–light petrol-
eum (b.p. 60–80 ЊC) at 3 ЊC afforded bright yellow crystals of
1,1-(PEt3)2-2,4-Ph2-7-OEt-1,2,4-closo-PtC2B9H8 5b, mass 0.047
g (38%) (Found: C, 43.3; H, 6 95. Calc. for C28H53B9OP2Pt: C,
44.3; H, 7.05%). NMR: 1H, δ 7.38 (m, 4 H, Ph), 3.50 [d of q, 1
H, OCH2, J(HH) 10, 6], 3.02 [d of q, 1 H, OCH2, J(HH) 10, 6
Hz], 1.97 (m, 6 H, PCH2), 1.58 (m, 6 H, PCH2), 1.12–0.90
(m, 12 H, PCH2CH3 and CH3) and 0.38 (m, 9 H, PCH2CH3);
11B-{1H},¶ δ 0.44 (1 B), Ϫ4.49 (2 B), Ϫ6.60 (1 B), Ϫ16.25 (3 B)
and Ϫ20.05 (2 B); 31P-{1H}, δ 4.21 [d, J(PP) 7, J(PtP) 4707] and
Ϫ7.52 [d br, J(PtP) 3678 Hz].
NMR spectrum of 1a at 400 MHz shows, as well as resonances
due to phenyl groups and the triethylammonium counter ion,
an apparent quartet centred at δ 3.67 of integral two and an
apparent triplet at δ 1.07, integral three, suggesting the presence
of a cage-bound ethoxide group. Nine inequivalent resonances
(some overlapping) were observed in the 11B-{1H} NMR spec-
trum between δ 2.0 and Ϫ35.8, the region associated with
{nido-C2B9} fragments. On retention of proton coupling the
highest-frequency signal remained a clear singlet, indicating
substitution was at one of the boron vertices. However, the site
of substitution was unclear and a single-crystal X-ray diffrac-
tion experiment was performed on 1a.
A perspective view of compound 1a, demonstrating the
atomic numbering scheme adopted, in shown in Fig. 1, whilst
Table 2 lists selected bond distances and angles. Compound 1a
has a nido molecular architecture with an ethoxide group
appended to B(3) on the lower pentagonal belt of the C2B9 cage.
The carbaborane cage has B᎐B bond distances ranging between
1.746(9) and 1.833(8) Å and B᎐C bond lengths between
1.625(7) and 1.724(7) Å. These distances can be compared with
those found in 8-SMe2-7,9-closo-C2B9H11,9 1.748(6)–1.841(6)
and 1.583(5)–1.693(6) Å respectively.
The two phenyl substituents on the cage carbon atoms are in
different orientations with respect to the cage. In closo 1,2-
diphenylcarbaboranes (and derivatives thereof)10 the phenyl
ring conformation has been described by the parameter θ, the
modulus of the average Ccage᎐Ccage᎐Cphenyl᎐Cphenyl torsion angle.
However, in the systems described here this parameter is no
longer applicable as the cage carbon atoms are not adjacent.
Instead, the twist of the phenyl rings is described by the par-
ameter γ, the dihedral angle between the mean plane of the
phenyl ring carbon atoms and the plane defined by the cage
carbon atom and the two boron atoms directly in line with it
when viewed down the Ccage᎐Cipso-P bond [i.e. C(9), B(1) and
B(2)]. Using this notation values of γ may be loosely compared
with values of θ. Thus, for the phenyl substituent on C(9), the
aromatic ring is twisted with respect to the plane defined by
C(9)᎐B(1)᎐B(2), to γ = 13.7Њ, lying essentially orthogonal to the
open C2B3 face, as is the situation found in [NEt3H][7,8-Ph2-7,8-
nido-C2B9H10]11 where the θ value is 7.8Њ averaged over both
phenyl rings. However, for the aryl ring attached to C(7) the
proximity of the ethoxide group means that the ring is rotated
to minimise steric interactions, the aromatic ring twisting with
Crystallography
All measurements were carried out at room temperature
on a Siemens P4 diffractometer, equipped with graphite-
monochromated Mo-Kα X-radiation (λ = 0.710 73 Å). Crystal-
lographic data are given in Table 1. Periodic remeasurement of
three standard reflections revealed no significant crystal decay
or electronic instability in each case. Intensity data were cor-
rected for the effects of X-ray absorption by ψ scans.
The structures were solved without difficulty by direct
methods and refined by full-matrix least squares, using
SHELXTL.8 For compound 1a all cage H atoms were located
and postionally refined with individual isotropic thermal
parameters. One quarter of a molecule of CH2Cl2 cocrystallises
per molecule of 1a, with 0.5 of a Cl atom per asymmetric unit
located ca. 1.5 Å from a centre of inversion; the associated C
and H atoms could not be located. In 1a the N᎐H distance was
restrained to 0.90 Å and the thermal parameter of this H was
fixed at 0.08 Å2. For compounds 2 and 4 cage H atoms were
placed in calculated positions, set 1.10 Å from B on a radial
extension. Owing to the disorder present in 2, the bridging H
atom on the B(10)᎐B(11) edge and the hydroxyl hydrogen were
not located. For all three structure determinations methyl,
methylene and phenyl H atoms were constrained to idealised
positions (C᎐H 0.97 for CH3 and CH2, 0.93 Å for aryl H) and
given isotropic displacement parameters riding at 1.2U(C). All
non-H atoms were refined with anisotropic displacement
parameters. In the final stages of refinement data were weighted
2
such that wϪ1 = [σ2(Fo) ϩ (g1P)2 ϩ g2P] where P = [max(Fo or
2
0) ϩ 2Fc ]/3.
Atomic coordinates, thermal parameters, and bond lengths
and angles have been deposited at the Cambridge Crystallo-
graphic Data Centre (CCDC). See Instructions for Authors,
J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the
CCDC for this material should quote the full literature citation
and the reference number 186/408.
Results and Discussion
Synthesis of [7,9-Ph2-7,9-nido-C2B9H10]Ϫ
The target carbaborane fragment for the synthesis of 1,1-
(PR3)2-2,4-Ph2-1,2,4-closo-PtC2B9H9 is [7,9-Ph2-7,9-nido-C2B9-
H10]Ϫ, the synthesis of which was initially attempted by base
Fig. 1 Perspective view of compound 1a. Thermal ellipsoids are
drawn at the 30% probability level, except for H atoms which have an
artificial radius of 0.1 Å for clarity. Phenyl rings are numbered cyclically
and H atoms of the phenyl groups carry the same number as the atom
to which they are attached
¶ Broadness of peaks means that J(BH) was not resolved except for the
resonance at δ Ϫ20.05 [J(BH) 109 Hz].
J. Chem. Soc., Dalton Trans., 1997, Pages 1205–1212
1207

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Table 1 Crystallographic data and details of data collection and structure refinement for compounds 1a, 2 and 4
1a
2
4
Formula
M
C22H40NOؒ0.25CH2Cl2
453.07
C30H56B9NO
544.05
C30H41B9P2Pt
755.95
System
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/n
a/Å
b/Å
c/Å
β/Њ
15.806(2)
12.4146(14)
14.549(4)
98.691(11)
2822.0(8)
4
1.066
0.104
15.727(2)
10.5290(11)
20.479(3)
93.386(13)
3385.4(7)
4
1.067
0.058
10.2034(9)
19.410(2)
16.670(2)
94.160(7)
3292.9(5)
4
U/Å
Z
Dc/g cmϪ3
1.525
4.379
µ(Mo-Kα)/mmϪ1
F(000)
970
1184
1496
θorientation/Њ
θdata collection/Њ
5.1–12.1
1–25
3.3–11.1
1–25
3.7–12.9
2–25
hkl Ranges
Ϫ18 to 18, 1 to Ϫ14, Ϫ1 to 17
1.5–40
6054
Ϫ1 to 18, Ϫ1 to 12, Ϫ24 to 24
1.5–30
7359
Ϫ1 to 12, Ϫ1 to 23, Ϫ19 to 19
1.5–60
7232
ω-Scan speed/Њ minϪ1
Data measured
Unique data
4898
5902
5755
g1
g2
0.2494
0.66
0.1414
4.31
0.0385
10.52
R (all data)
Data observed [Fo > 4σ(Fo)]
0.1727
2682
0.2301
2288
0.0638
4329
R (observed data)
wR2
S
0.1130
0.3245
1.072
0.0984
0.2472
0.994
0.0396
0.0993
1.097
Variables
346
1.538, Ϫ0.260
385
0.831, Ϫ0.296
379
0.569, Ϫ1.169
Maximum, minimum residue/e ÅϪ3
Table 2 Bond lengths (Å) and selected interbond angles (Њ) in compound 1a
O᎐C(51)
1.413(6)
1.750(8)
1.799(8)
1.686(7)
1.775(8)
1.724(7)
1.815(7)
1.766(8)
O᎐B(3)
1.417(6)
1.746(9)
1.792(8)
1.805(8)
1.761(8)
1.813(8)
1.771(8)
1.694(6)
B(5)᎐C(9)
B(5)᎐B(10)
B(6)᎐B(10)
C(7)᎐B(8)
B(8)᎐C(9)
C(9)᎐B(10)
C(51)᎐C(52)
1.670(7)
1.824(8)
1.799(8)
1.626(7)
1.625(7)
1.647(7)
1.454(9)
B(4)᎐B(8)
B(5)᎐B(6)
1.806(7)
1.807(9)
1.784(9)
1.507(7)
1.666(7)
1.511(6)
1.833(8)
B(1)᎐B(5)
B(1)᎐B(4)
B(2)᎐C(7)
B(2)᎐B(6)
B(3)᎐C(7)
B(3)᎐B(8)
B(4)᎐B(5)
B(1)᎐B(2)
B(1)᎐B(6)
B(1)᎐B(3)
B(2)᎐B(3)
B(2)᎐B(11)
B(3)᎐B(4)
B(4)᎐C(9)
B(6)᎐B(11)
C(7)᎐C(71)
C(7)᎐B(11)
C(9)᎐C(91)
B(10)᎐B(11)
C(51)᎐O᎐B(3)
B(5)᎐B(1)᎐B(6)
B(2)᎐B(1)᎐B(3)
C(7)᎐B(2)᎐B(3)
B(1)᎐B(2)᎐B(6)
B(6)᎐B(2)᎐B(11)
O᎐B(3)᎐B(2)
O᎐B(3)᎐B(4)
B(2)᎐B(3)᎐B(1)
O᎐B(3)᎐B(8)
B(4)᎐B(3)᎐B(8)
B(3)᎐B(4)᎐B(1)
C(9)᎐B(4)᎐B(8)
C(9)᎐B(5)᎐B(4)
B(1)᎐B(5)᎐B(6)
B(6)᎐B(5)᎐B(10)
120.6(4)
61.4(3)
59.4(3)
60.0(3)
61.2(3)
59.6(3)
119.4(4)
125.3(4)
58.6(3)
129.4(4)
60.5(3)
60.7(3)
55.2(3)
59.0(3)
60.5(3)
59.4(3)
B(2)᎐B(1)᎐B(6)
B(5)᎐B(1)᎐B(4)
B(4)᎐B(1)᎐B(3)
B(1)᎐B(2)᎐B(3)
C(7)᎐B(2)᎐B(11)
O᎐B(3)᎐C(7)
60.2(3)
59.7(3)
58.9(3)
62.0(3)
56.8(3)
121.5(4)
57.8(3)
123.9(4)
60.4(3)
54.6(3)
57.7(3)
58.8(3)
61.0(3)
61.6(3)
56.0(3)
61.3(3)
B(2)᎐B(6)᎐B(1)
B(1)᎐B(6)᎐B(5)
C(71)᎐C(7)᎐B(8)
C(71)᎐C(7)᎐B(2)
C(71)᎐C(7)᎐B(3)
B(2)᎐C(7)᎐B(3)
C(7)᎐B(8)᎐B(3)
C(91)᎐C(9)᎐B(8)
C(91)C(9)᎐B(5)
B(10)᎐C(9)᎐B(5)
B(8)᎐C(9)᎐B(4)
C(9)᎐B(10)᎐B(5)
B(6)᎐B(10)᎐B(11)
B(6)᎐B(11)᎐B(2)
O᎐C(51)᎐C(52)
58.6(3)
58.2(3)
121.1(4)
120.2(4)
116.7(4)
62.2(3)
59.9(3)
121.0(4)
119.1(4)
66.7(3)
65.9(3)
57.2(3)
58.8(3)
59.1(3)
114.0(5)
B(11)᎐B(6)᎐B(10)
B(10)᎐B(6)᎐B(5)
C(71)᎐C(7)᎐B(11)
B(11)᎐C(7)᎐B(2)
B(8)᎐C(7)᎐B(3)
C(9)᎐B(8)᎐B(4)
B(4)᎐B(8)᎐B(3)
C(91)᎐C(9)᎐B(10)
B(8)᎐C(9)᎐B(5)
C(91)᎐C(9)᎐B(4)
B(5)᎐C(9)᎐B(4)
B(6)᎐B(10)᎐B(5)
C(7)᎐B(11)᎐B(2)
B(6)᎐B(11)᎐B(10)
61.5(3)
60.8(3)
119.6(4)
65.5(3)
65.5(3)
58.9(3)
58.6(3)
118.3(4)
113.5(4)
117.5(4)
63.3(3)
59.8(3)
57.8(3)
59.6(3)
C(7)᎐B(3)᎐B(2)
O᎐B(3)᎐B(1)
B(4)᎐B(3)᎐B(1)
C(7)᎐B(3)᎐B(8)
C(9)᎐B(4)᎐B(5)
B(5)᎐B(4)᎐B(1)
B(3)᎐B(4)᎐B(8)
B(1)᎐B(5)᎐B(4)
C(9)᎐B(5)᎐B(10)
B(2)᎐B(6)᎐B(11)
respect to C(7)᎐B(1)᎐B(5), to γ = 79.1Њ. The B(3)᎐O bond
length is 1.417(6) Å, which is in the range for such bonds,12 and
the B᎐B connectivities to B(3) are not significantly distorted.
The molecule crystallises with one close intermolecular contact
with the [NEt3H]ϩ counter ion, a hydrogen bond being present in
the solid state between O and H(90) at 1.915 Å.
NMR spectrum displays at least seven overlapping inequivalent
boron environments between δ 0.53 and Ϫ39.05, suggesting
that in 1b the ethoxide is substituted at either B(2) or B(11),
assuming that the cage carbons have not undergone rearrange-
ment. However, the shielding pattern of a {7,9-Ph2-7,9-nido-
C2B9} fragment substituted at B(11) (see below) is different to
that observed for 1b. Thus we assign 1b as [NEt3H][2-OEt-7,9-
Ph2-7,9-nido-C2B9H9].
With the result of the structure determination of compound
1
1a thus established, re-examination of the H NMR spectrum
shows that, although the methylene protons on the ethoxide
group are chemically inequivalent, they are apparently suf-
ficiently magnetically similar to be observed as only one appar-
ent quartet (even at 400 MHz). This is not the case, however, for
the minor product, 1b. For this species the ethoxide substituent
is observed as two quartets at δ 3.59 and δ 3.42, both integrat-
ing to one proton each and a double of doublets (apparent
triplet) at δ 0.93, equivalent to three protons. The 11B-{1H}
It has previously been reported that, under similar conditions
to those employed here, 1,7-Me2-1,7-closo-C2B10H10 undergoes
base degradation to afford an alkoxy-substituted 7,9-Me2-7,9-
nido-C2B9 cage.13 On the basis of chemical and NMR experi-
ments, the position of the alkoxy substituent was determined to
be at B(3), consistent with the solid-state structure reported
here for compound 1a. However, no other isomeric form (e.g.
substitution in the 2 position) was reported.
1208
J. Chem. Soc., Dalton Trans., 1997, Pages 1205–1212

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Table 3 Bond lengths (Å) and selected interbond angles (Њ) in compound 2
B(1)᎐B(2)
B(1)᎐B(3)
B(1)᎐B(4)
B(2)᎐B(3)
B(2)᎐B(11)
B(3)᎐B(4)
B(4)᎐C(9)
B(4)᎐B(8)
1.750(11)
1.775(9)
B(1)᎐B(5)
B(1)᎐B(6)
B(2)᎐C(7)
B(2)᎐B(6)
B(3)᎐C(7)
B(3)᎐B(8)
B(4)᎐B(5)
B(5)᎐C(9)
1.754(10)
1.783(13)
1.676(9)
1.786(10)
1.684(9)
1.830(12)
1.780(10)
1.665(9)
B(5)᎐B(6)
1.747(12)
1.788(11)
1.501(7)
1.665(8)
1.495(8)
1.375(9)
1.356(13)
B(5)᎐B(10)
B(6)᎐B(11)
C(7)᎐B(8)
B(8)᎐C(9)
C(9)᎐B(10)
B(10)᎐B(11)
1.789(12)
1.813(11)
1.607(10)
1.580(9)
1.659(9)
1.812(10)
B(6)᎐B(10)
C(7)᎐C(71)
C(7)᎐B(11)
C(9)᎐C(91)
B(10)᎐O(1A)
B(11)᎐O(1B)
1.789(10)
1.764(11)
1.827(11)
1.780(10)
1.702(9)
1.841(12)
B(2)᎐B(1)᎐B(3)
B(5)᎐B(1)᎐B(6)
B(3)᎐B(1)᎐B(4)
B(1)᎐B(2)᎐B(3)
C(7)᎐B(2)᎐B(11)
C(7)᎐B(3)᎐B(2)
B(1)᎐B(3)᎐B(4)
B(4)᎐B(3)᎐B(8)
B(5)᎐B(4)᎐B(1)
C(9)᎐B(4)᎐B(8)
B(6)᎐B(5)᎐B(1)
B(1)᎐B(5)᎐B(4)
B(6)᎐B(5)᎐B(10)
B(1)᎐B(6)᎐B(2)
B(2)᎐B(6)᎐B(11)
60.1(4)
59.2(5)
59.9(4)
60.6(4)
56.6(4)
58.1(4)
60.4(4)
61.3(4)
58.9(4)
52.8(4)
61.2(5)
60.8(4)
60.7(5)
58.7(5)
61.0(4)
B(2)᎐B(1)᎐B(6)
B(5)᎐B(1)᎐B(4)
C(7)᎐B(2)᎐B(3)
B(1)᎐B(2)᎐B(6)
B(6)᎐B(2)᎐B(11)
B(2)᎐B(3)᎐B(1)
C(7)᎐B(3)᎐B(8)
C(9)᎐B(4)᎐B(5)
B(3)᎐B(4)᎐B(1)
B(3)᎐B(4)᎐B(8)
C(9)᎐B(5)᎐B(4)
C(9)᎐B(5)᎐B(10)
B(5)᎐B(6)᎐B(1)
B(5)᎐B(6)᎐B(10)
B(10)᎐B(6)᎐B(11)
60.7(4)
60.3(4)
58.6(4)
60.5(5)
60.2(4)
59.3(4)
54.2(4)
57.1(4)
59.6(4)
60.7(4)
59.1(4)
57.3(4)
59.6(5)
60.8(4)
60.4(4)
C(71)᎐C(7)᎐B(8)
C(71)᎐C(7)᎐B(2)
B(8)᎐C(7)᎐B(3)
C(7)᎐B(8)᎐B(3)
B(3)᎐B(8)᎐B(4)
C(91)᎐C(9)᎐B(10)
B(10)᎐C(9)᎐B(5)
B(8)᎐C(9)᎐B(4)
O(1A)᎐B(10)᎐C(9)
O(1A)᎐B(10)᎐B(5)
B(6)᎐B(10)᎐B(5)
B(6)᎐B(10)᎐B(11)
O(1B)᎐B(11)᎐B(6)
B(6)᎐B(11)᎐B(10)
C(7)᎐B(11)᎐B(2)
122.7(5)
120.2(5)
67.5(5)
58.3(4)
58.0(4)
119.7(5)
65.1(5)
68.2(5)
119.1(6)
108.7(7)
58.5(5)
60.5(4)
118.6(8)
59.1(4)
57.2(4)
C(71)᎐C(7)᎐B(11)
B(11)᎐C(7)᎐B(2)
B(2)᎐C(7)᎐B(3)
117.7(5)
66.3(4)
63.3(4)
C(9)᎐B(8)᎐B(4)
59.1(4)
C(91)᎐C(9)᎐B(8)
C(91)᎐C(9)᎐B(5)
C(91)᎐C(9)᎐B(4)
B(5)᎐C(9)᎐B(4)
O(1A)᎐B(10)᎐B(6)
C(9)᎐B(10)᎐B(5)
O(1A)᎐B(10)᎐B(11)
O(1B)᎐B(11)᎐C(7)
O(1B)᎐B(11)᎐B(10)
O(1B)᎐B(11)᎐B(2)
B(6)᎐B(11)᎐B(2)
120.5(5)
122.0(5)
117.4(5)
63.8(4)
116.4(7)
57.6(4)
131.8(7)
122.8(7)
128.1(7)
114.3(8)
58.8(4)
As we are interested in the consequences of treating [7,9-Ph2-
7,9-nido-C2B9H9]2Ϫ with [PtCl2(PMe2Ph)2], the associated im-
plications of cage phenyl-induced distortions and, specifically,
comparison of the products of such a reaction with those
from [7,8-Ph2-7,8-nido-C2B9H9]2Ϫ, the presence of an appended
ethoxide group on the cage of compounds 1a and 1b was un-
wanted, and an alternative route to cage degradation was
sought.
Treatment of 1,7-Ph2-1,7-closo-C2B10H10 with 5 equivalents
of [NBu4] (thf solution, ca. 5% water) in thf at room temper-
ature gave no reaction. However, heating the reaction mixture to
reflux for 18 h resulted in complete conversion of the starting
material into a nido product 2. The 11B-{1H} NMR spectrum
showed that the product was not the anticipated mirror sym-
metric [7,9-Ph2-7,9-nido-C2B9H10]Ϫ species, displaying nine
inequivalent boron environments in the range δ 3.4 to Ϫ33.7,
with a shielding pattern significantly different from that found
in 1a or 1b. The 11B NMR spectrum indicated substitution at
one of the boron vertices, the resonance at δ 3.67 remaining a
singlet on proton coupling. Positive-ion FAB mass spec-
trometry and microanalysis suggested the formulation of 2 to
be [NBu4][C2Ph2B9H9X], where X was either OH or F. Fluoride
substitution on the cage (the opening of a closo-C2B6 cluster to
afford a nido-species with fluoride incorporated having previ-
ously been reported 14) was initially ruled out by the absence of
a signal in the 19F NMR spectrum. Additionally, a broad peak
in the IR spectrum, centred at 3443 cmϪ1 was assigned to an OH
group stretch. A single-crystal X-ray diffraction study was car-
ried out on compound 2 in order to attempt to confirm the pos-
ition and identity of this substituent.
Fig. 2 Perspective view of the anion of compound 2. Construction
and labelling conventions as in Fig. 1
16
C2B9H6 and 1.406(4) Å in 8-OH-9-Me-6-nido-CB10H10.17 No
indication of significant B᎐B connectivity lengthening around
the B᎐OH bond was observed,15,16 as has previously been noted
in closo-hydroxyl-substituted carbaborane systems.
Unfortunately, the solid-state structural determination of
compound 2 does not allow the distinction between OH and F,
and a degree of caution must be employed in the assignment of
the appended group. Noteworthy, though, is that water is
strongly intimated in the mechanism of fluoride-induced
deboronation.14
The inability of either base- or fluoride-induced deboron-
ation to afford the required product led us to consider a different
method.18 Thus, we attempted to isomerise Cs[7,8-Ph2-7,8-nido-
C2B9H10] by heating a sample in a sealed tube for 4 h at 300 ЊC,
A perspective view of a single molecule of compound 2 show-
ing the atom labelling scheme is shown in Fig. 2, with salient
bond lengths and angles given in Table 3. The molecular archi-
tecture is nido-C2B9, with a single hydroxyl group appended to
one of the symmetry-equivalent boron atoms of the open C2B3
face. The hydroxyl group is crystallographically disordered
between O(1A) and O(1B), the occupancy at O(1A) being
60.51(6)%. The aromatic cage substituents on C(7) and C(9) lie
essentially orthogonal to the C2B3 face (γ = 19.6 and 6.1Њ
respectively). The B᎐B bonds lengths range between 1.747(12)
and 1.841(12) Å, with the C᎐B lengths 1.580(9)–1.702(9) Å. The
B(10)᎐O(1A) and B(11)᎐O(1B) lengths are 1.375(9) and
1.356(13) Å respectively (average 1.365 Å). These compare with
following
a
method described previously.7 The product
recovered displayed a 11B-{1H} NMR spectrum which demon-
strated that the molecule possessed a mirror plane of symmetry,
with only five resonances observed (δ 0.7 to Ϫ32.8) in the ratio
3:2:2:1:1 (the first signal coincident); this spectrum is clearly
not that of the 7,8-C2B9 starting material. Analysis of a 11B᎐11B
(correlation spectroscopy, COSY) spectrum confirmed the
identity of the product as Cs[7,9-Ph2-7,9-nido-C2B9H10]
(Scheme 1), which was converted into the required [NEt3H]ϩ
salt 3 by simple metathesis.
Reaction with [PtCl2(PMe2Ph)2]
1.384(5)
Å
in 6-Me2S-4-OH-2-(Me3Si)2CH-closo-CB10H8,15
Deprotonation of compound 3, by heating to reflux in thf
1.35(1) and 1.37(1) Å in 2,3-Me2-4,7-(OH)2-10-Br-2,3-closo-
with an excess of sodium hydride, and subsequent reaction with
J. Chem. Soc., Dalton Trans., 1997, Pages 1205–1212
1209

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